As a drive mechanism employing an electromechanical transducer such as a piezoelectric element which varies in length (i.e. expands and contracts) with application of voltage, there has been conventionally provided a drive mechanism as shown, for example, in an exploded perspective view of FIG. 1, and in an assembly perspective view of FIG. 2.
This drive mechanism, in which a moving part 10 can be moved relative to a base seat (anchor block) 1 as will be described later, can be used as, for example, a lens drive mechanism in a camera. That is, in case that the moving part 10 is connected to a lens frame (unshown), a lens provided in the lens frame can be moved along with the movement of the moving part 10.
A piezoelectric element 4 is constituted by stacking or laminating a large number of piezoelectric plates. One end 4a of the piezoelectric element 4 in its expanding/contracting direction is fixed to the base seat 1, while the other end 4b thereof is fixed to one end 5a of a driving rod 5. The rod 5 is slidably supported by support portions 2 and 3 formed integrally with the base seat 1.
In the moving part 10, the rod 5 is held or sandwiched between a body 11 and a cap 12, and a biasing force in its holding direction of the rod 5 is exerted upon the body 11 and the cap 12 by a pressure spring 13. Thus, the body 11 and the cap 12, of the moving part 10, frictionally hold the rod 5.
An unshown voltage control circuit (drive pulse generating device) is connected to the piezoelectric element 4. When a fluctuating voltage in a sawtoothed wave form as shown in FIG. 3 is continuously applied to the piezoelectric element 4 from the voltage control circuit, the piezoelectric element 4 expands and contracts, causing the rod 5 to oscillate or vibrate in its lengthwise direction.
More specifically, when the piezoelectric element 4 expands relatively slowly in accordance with a gently rising slope portion 101 of a first wave 100 of the input voltage as shown in FIG. 3, the rod 5 moves slowly in a direction of arrow A relative to the base seat 1 as shown in FIG. 2. Then, when the voltage is abruptly dropped down as shown by an abruptly (or steeply) falling slope portion 102 of the first wave 100 in the same figure, the piezoelectric element 4 abruptly contracts to return back to the initial length. As a result, the rod 5 abruptly moves in a direction of arrow B relative to the base seat 1.
When the following waves 100', 100" . . . , similar to the first wave 100, of the input voltage, as shown in FIG. 3, are continuously applied to the piezoelectric element 4, the rod 5 repeatedly oscillates or vibrates so that the rod 5 moves slowly in the direction A and moves abruptly in the direction B.
The spring force of the pressure spring 13 of the moving part 10, or the frictional force exerted between the rod 5 and the moving part 10, is adjusted so that the moving part 10 moves together with the rod 5 relative to the base seat 1 when the rod 5 moves slowly, and so that the moving part 10 remains stationary relative to the base seat 1 when the rod 5 rapidly or abruptly moves relative thereto. Accordingly, while the rod 5 is being oscillated, the moving part 10 moves in the direction A relative to the base seat 1.
Meanwhile, the moving part 10 can be moved in a direction B in FIG. 2, by changing the pulse wave form of the input voltage applied to the piezoelectric element 4 into a wave form having an abruptly (or steeply) rising slope portion and a gently falling slope portion, instead of the wave form shown in FIG. 3. The principle that the moving part 10 is moved or actuated in this case, is similar to the principle of the foregoing.
With the constitution of the drive mechanism using the piezoelectric element 4 shown in FIGS. 1 and 2, the amplitude and frequency of the vibrating rod 5 are decided by the maximum value and frequency of the applied pulse voltage, provided that the same piezoelectric element is used.
Also, with the constitution thereof, the greater the amplitude and the higher the frequency of the rod 5 are, the greater the relative moving speed between the rod 5 and the moving part 10 is.
Therefore, in order to attain a higher speed of actuator by increasing the moving speed of the moving part 10 with the same frequency of the applied pulse voltage, it is inevitably necessary to increase the maximum value of the pulse voltage applied to the piezoelectric element 4.